Generally, a polymer electrolyte fuel cell includes a membrane-electrode assembly which, in turn, includes an anode electrode layer formed on one side thereof and a cathode electrode layer formed on the other side thereof. Fuel gas (e.g., hydrogen gas) and oxidizer gas (e.g., air) are supplied to the anode electrode layer and the cathode electrode layer, respectively, via respective separators. Supply of fuel gas and oxidizer gas to the anode electrode layer and the cathode electrode layer induces electrode reactions in the membrane-electrode assembly, thereby generating electricity. Electricity generated in the membrane-electrode assembly is output to the exterior of the fuel cell via the separators.
The polymer electrolyte fuel cell generates electricity efficiently by means of efficiently supplying fuel gas and oxidizer gas to the membrane-electrode assembly; more specifically, to the anode electrode layer and the cathode electrode layer, so as to accelerate electrode reactions in the membrane-electrode assembly. Efficient collection of electricity generated in the membrane-electrode assembly allows efficient output of generated electricity to the exterior of the fuel cell.
In order to achieve the above-mentioned efficient generation of electricity, the polymer electrolyte fuel cell employs separators which are formed of a metal sheet impermeable to gas and on which a large number of streaky recesses and projections are formed. In the conventional separators, externally supplied fuel gas and oxidizer gas flow through streaky recess portions (or projection portions) thereof to thereby be supplied to the anode electrode layer and the cathode electrode layer. Thus, limitations are imposed on a contact area between supplied fuel gas and the anode electrode layer and that between supplied oxidizer gas and the cathode electrode layer, possibly resulting in a failure to sufficiently supply fuel gas and oxidizer gas. By contrast, a contact area between the anode electrode layer and the corresponding separator and that between the cathode electrode layer and the corresponding separator are provided in excess, so that the separators can collect generated electricity with very high efficiency.
As mentioned above, a separator which has conventionally been employed in the polymer electrolyte fuel cell is inferior in terms of efficient supply of gas, but is superior in terms of efficient collection of electricity. In other words, gas supply efficiency and electricity collection efficiency are not appropriately balanced.
In order to cope with the above problem, for example, Patent Document 1 discloses a separator for a fuel cell which exhibits improved gas supply efficiency. This separator for a fuel cell includes a flat-sheet-like first member (carbon) and a second member (sheet metal) which is superposed on the first member and has a plurality of protruding pieces to elastically contact an anode electrode layer or a cathode electrode layer and to form a gas passageway. The gas passageway formed by means of the plurality of protruding pieces of the second member assume the form of spaces around and behind the protruding pieces, thereby allowing externally supplied fuel gas or oxidizer gas to pass therethrough in every direction.
Thus, the disclosed separator can well diffuse fuel gas or oxidizer gas and can enhance efficiency in gas supply to the anode electrode layer or the cathode electrode layer. In the separator, the protruding pieces of the second member come into surface contact with the anode electrode layer or the cathode electrode layer; thus, the separator can collect generated electricity efficiently. The separator outputs generated electricity to the exterior of the fuel cell via the first member.
The disclosed conventional separator for a fuel cell can well diffuse fuel gas or oxidizer gas. However, since the protruding pieces of the second member are actively brought into surface contact with the anode electrode layer or the cathode electrode layer by utilization of elasticity, a contact area between diffused fuel gas and the anode electrode layer or between diffused oxidizer gas and the cathode electrode layer is reduced. In other words, the area of surface portions of the anode electrode layer or the cathode electrode layer covered with the second member increases, possibly resulting in a failure to supply gas to the anode electrode layer or the cathode electrode layer in an amount required for an electrode reaction. Thus, in this respect, there remains room for improvement.
In order to cope with the above problem, for example, Patent Document 2 discloses a fuel cell in which gas supply efficiency is improved. In this fuel cell, fuel gas and oxidizer gas are supplied to an anode electrode layer and a cathode electrode layer, respectively, via a porous metal (nickel foam). This reduces a contact area between the anode electrode layer and the corresponding porous metal and that between the cathode electrode layer and the corresponding porous metal, thereby allowing supply of gas in an amount required for an electrode reaction. Also, the anode electrode layer and the surface of the corresponding porous metal contact each other, and the cathode electrode layer and the surface of the corresponding porous metal contact each other, thereby securing a contact area required for efficiently collecting generated electricity. Thus, electricity collection efficiency is improved.
However, generally, the cost of manufacturing porous metal is very high; thus, the cost of manufacturing a fuel cell which employs porous metal increases. Therefore, there is urgent demand for development of a separator which enhances gas supply efficiency and electricity collection efficiency and whose manufacturing cost is low.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2002-184422
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 07-22037